A. Field of the Invention
This invention relates to data communication equipment.
B. Related Art
Apparatus which perform data communication operations over telephony lines are well known in the art. For example, it is known to provide a digital telephone with a data communication controller (DCC) which enables an attached computer to communicate with the digital telephone itself, or with another device (e.g. another computer) by way of a digital telephony line. Where data communication over an analog telephony line is desired, a modulator-demodulator (more commonly known as a "modem") is used to convert the digital computer generated signals into modulated analog signals for transmission over the analog telephony lines. A modem at a receiving station demodulates the analog signals and converts them back into digital form for use by a digital device at the receiving end of the telephony line.
DCCs and modems typically have two modes of operation: a "transparent mode" in which digital data from the sending device is passed through to the telephony lines (in analog form in the case of a modem), and a "command mode" in which the DCC or modem will accept commands from the sending device. For example, while in command mode the sending device may have the DCC dial a number or select an incoming or outgoing line on the digital telephone.
The prior art includes a number of ways of switching between transparent mode and command mode. For example, it is known in the art to have a modem switch from transparent mode to command mode upon recognition of a predefined escape sequence. The escape sequence may take a number of forms. For example, the escape sequence may take the form a non-printable character, such as "Ctrl-X" (the "X" key depressed along with the "Ctrl" key).
One problem with such escape sequences is that, while in transparent mode, if a sequence of characters matching the escape sequence happens to appear in the transmitted character stream, the DCC or modem may undesirably or unexpectedly change from the transparent mode to the command mode. Thus, in DCCs or modems where it is desired to transmit the escape sequence characters across the link while in transparent mode, special provision is made to distinguish a reception (from the computer) of the escape character sequence which is intended to be retransmitted as data, from reception of the same escape character sequence which is intended as a signal for the DCC or modem to switch from transparent mode to command mode.
The prior art includes a number of mechanisms for delimiting the "true" escape sequence from a sequence of characters to be retransmitted. For example, in synchronous protocols it is known to precede the escape sequence by predefined delimiter characters. For asynchronous transmissions, it is known in the art to provide a modem with a mechanism that escapes from transparent mode, if and only if the escape sequence is preceded and/or followed by a predetermined time period during which no characters are transmitted. Some escape mechanisms add the additional constraint that (in order to cause an escape) the characters in the escape sequence must arrive separated from one another in time by less than a predetermined time.